Gas electrodes, in and with which a gas is passed in contact with a suitable electrode conductor in the presence of an electrolyte solution, are well known. Many modern gas electrodes are made to be porous and to have catalytically-active surface areas. These include those wall surface areas of the pores disposed internally within the electrode body. In this way, there can be realized maximization of the available and effective surface area of the electrode for given unit geometric volumes of the electrode configuration.
Such general type and style of electrode construction is especially advantageous for the oxygen gas-bearing electrodes that are well adapted for the electroreduction of oxygen in alkaline media.
The usage technique applied with such electrodes often involves passage of the oxygen-bearing gas through the porous electrode body for contact with the involved electrolyte interstitially therewithin and/or at and on the electrolyte-contacting face or wall of the electrode body. The indicated practice is desirable for electrolyzing functions and, conversely as well, for operations in the galvanic mode as in fuel cells. Oxygen gas-bearing depolarized cathodes so made and operated are particularly attractive for utilization in chlor-alkali and the like or equivalent manufacturing cell operations.
A great and impelling reason (although other benefits also accrue) for employing oxygen gas-bearing, depolarized porous electrodes to electrolyze common salt brine into chlorine and caustic soda (i.e., NaOH) and for analogous production purposes, is pure and simple economics. Potentially very impressively significant savings in power requirements for given electrolysis workings are anticipatable due to substantial reductions achievable in needs for applied electrical potential when such electrodes are utilized. This is evident in comparison of operating voltage levels for the involved electrochemical reactions, taking into account that conventional cells already are usually operated at quite low voltages; the cathodic reactions (disregarding overvoltage effects) respectively being:
In traditionally common chlor-alkali cells: EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.-, (I)
with E.degree.=-0.828 volt; and PA1 with E.degree.=0.401 volt; PA1 P is the capillary pressure in atmospheres; PA1 .gamma. is the surface tension in dynes/cm; PA1 .theta. is the contact angle; and PA1 r is the pore radius in microns.
With the oxygen-gas depolarized cathodes: EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.-, (II)
there being a consequent theoretically attainable saving of 1.229 volts in the difference.
Literally from their inception and classically, oxygen electrodes have been catalyzed by various precious and semi-precious metals and compounds thereof, such as gold, osmium, palladium, platinum, silver and so forth, and their alloys, oxides and other compositions. These noble metals are not only in generally scarce supply for other than jewelry adornments and ornamentations and/or monetary purposes, but are inherently extremely expensive for industrial applications. Because of this, their consumption for electrode preparation is carefully controlled and extended to the greatest possible extent; this usually being done so as to minimize total quantity usage by deposition thereof in the form of platings or other applied layers or coatings over a suitable substrate, such as porous nickel plaque.
This last-mentioned possibility, at least superficially and ostensibly, would seem to have ensured the provision on an economically reasonable basis of reliable and effective porous electrodes that are optimumly electrocatalytically effective.
Surprisingly, however, the stated expectation is not the case. Satisfactory and effective pore depositions of precious and semi-precious mtals and their compounds and many other catalytic materials in porous electrode bodies is, nonetheless, not always easily or directly achievable; complex procedures and manipulations oftentimes being required for the purpose. Sometimes, in fact, the normal and ordinarily employed porous electrode bodies cannot be employed for such catalyzation; oftentimes requiring utilization of specially constructed and compositioned materials for their fabrication which may tend to actually be physically inferior as electrode units. In other instances they may even require special treatments or conditionings and particularized applicating procedures to possibilitate or enhance catalyst deposition.
To illustrate a commonly encountered difficulty along the above-mentioned lines, the most desirable range of average nominal pore size in electrode bodies in order to most effectively facilitate electrochemical reactions is generally from 1 (or sometimes even less) or so to about, say, 12 microns. This, for reasons hereinafter better explained, is especially so in connection with gas diffusion electrodes with which it appears and is believed to be most advantageous to effectuate the reaction interiorly within the porous body structure. In attempting by ordinary procedures to electroplate a precious or semi-precious metal catalyst within and on the enclosed wall surface(s) of such porules, it is frequently and on those occasions disadvantageously found that most of the metal catalyst tends to unavoidably be deposited on the external face wall of the electrode body. To avoid such frustrating ineffectualities, sophisticated and complex techniques have been developed including such strategems as pumping plating solution through the porous metal substrate as disclosed in U.S. Pat. No. 3,787,244 and Canadian Pat. No. 921,111. Analogous problems of pore blockage and inefficient and/or misplaced catalyst deposition are similarly encountered when attempting to coat or layer other electrocatalytic agents within such finely-pored electrode bodies.
It would obviously be desirable to easily and readily have some reliable and relatively problem-free way to catalyze and so provide the interior pore wall surfaces of very fine pore structured electrode bodies, especially when very costly platings of gold, palladium, platinum, silver and the like are involved and to have such catalyzed electrode products for superior performance in electrochemical reactions.